The sea bed includes surface layers of sedimentary material, exhibiting varying degrees of looseness or consolidation, which are composed of small particles of said, silt, and clay in, on and among which may be dispersed small particles of rock and metallic nodules. For the sake of convenience, the term "silt" is used in this disclosure and accompanying claims to refer to all such sedimentary material other than the nodules, including sand, clay, silt or small rock particles, or a combination thereof, irrespective of whether the sedimentary material is of a loose or agglutinated character.
Since at least the early 1960's it has been considered that literally billions of tons of such valuable metals as manganese, nickel, copper, and cobalt lie associated with such silt layers. Many of the nodules are believed to lie on the surface of the silt. Others are believed to lie partially or completely buried near the silt surface.
The presence of these nodules generated keen interest in development of dredging equipment for collecting them and raising them to the surface. However, the bulk of the proposals which have been published to date involve one or more disadvantages. These include one or more of the following: pressing the surface nodules and only partially buried nodules into the silt before collecting same, scooping up all of the material in the silt layer to a given depth even though only a portion of the layer is desired material, excessive resistance to transport over the sea bed, preferentially collecting the smallest nodules, choking up with oversize material, inadequate removal of silt from the nodules prior to lifting same to the surface, requirement of a separate unit for cleaning the nodules prior to such lifting, the energy consumption and maintenace costs and delays connected with the use of powerful electric motors and turboshaft engines in the hostile, inaccessible environment where the dredge operates. Thus, a need remains for improved apparatus and processes which avoid some or all of the foregoing disadvantages.